Evolutionary Biology Terms Starting With R
Evolutionary Biology Glossary: R
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Reciprocal Altruism
/ rih-SIP-ruh-kul AL-troo-iz-um / · Latin reciprocus meaning moving back and forth and alter meaning other
Reciprocal altruism is a form of cooperative behavior in which one individual helps another at a short-term cost to itself, with the benefit of receiving help in return during a future interaction.
The concept was formalized by Robert Trivers in 1971, who showed mathematically that helping can spread through a population when individuals interact repeatedly, recognize one another, and can detect and avoid cheaters. Vampire bats (Desmodus rotundus) provide one of the best-documented examples: a bat that fails to feed on a given night risks starvation within 60 hours, and well-fed roost-mates will regurgitate blood meals to hungry individuals. Gerald Wilkinson’s field studies in Costa Rica during the 1980s confirmed that bats preferentially share with individuals that have fed them before, and that this pattern holds even among non-relatives.
Cleaner wrasses (Labroides dimidiatus) on coral reefs show a parallel pattern, with client fish returning repeatedly to cleaners that provide reliable service rather than biting them.
Computer tournaments run by political scientist Robert Axelrod in the early 1980s found that a simple strategy called "tit-for-tat," cooperate on the first move and then copy whatever the partner did last, consistently outcompeted more elaborate strategies in iterated prisoner's dilemma games, giving a formal mathematical grounding to why reciprocal cooperation can be stable.
Reciprocal altruism is selfless generosity. The behavior spreads precisely because helping now statistically increases the helper's own future survival or reproduction, making it a form of delayed self-interest rather than true altruism.
Vampire bats (Desmodus rotundus) in Costa Rican colonies share regurgitated blood meals with roost-mates that failed to feed on a given night. Wilkinson's data showed that bats are roughly twice as likely to share with an individual that has shared with them before, and that this preference persists even after controlling for genetic relatedness.
Reproductive Isolation
/ REE-pruh-DUK-tiv eye-suh-LAY-shun / · Latin reproductio meaning producing again and insula meaning island
Reproductive isolation is the set of biological mechanisms that prevent two populations from exchanging genes, either by blocking mating attempts or by making any resulting hybrids sterile or inviable.
Biologists divide these mechanisms into prezygotic barriers, which act before fertilization, and postzygotic barriers, which act after. Prezygotic barriers include differences in habitat, mating season, courtship behavior, flower structure, and gamete compatibility; postzygotic barriers include hybrid inviability, hybrid sterility, and hybrid breakdown in later generations. Ernst Mayr placed reproductive isolation at the center of his Biological Species Concept in 1942, arguing that a species is defined by the existence of such barriers rather than by morphology alone.
Eastern and western meadowlarks (Sturnella magna and Sturnella neglecta) overlap broadly across the Great Plains yet rarely hybridize, because their songs differ enough that females do not respond to the calls of the other species, a behavioral prezygotic barrier operating without any physical separation.
Liger and tigon hybrids, produced by crossing lions (Panthera leo) and tigers (Panthera tigris) in captivity, are almost always sterile, illustrating postzygotic isolation between two species that would never encounter each other in the wild and therefore never needed to evolve prezygotic barriers.
Species are isolated only by physical distance. Two populations can be reproductively isolated while living in the same habitat if behavioral, temporal, or genetic barriers prevent successful interbreeding.
The Kaibab squirrel (Sciurus kaibabensis) on the north rim of the Grand Canyon and the Abert's squirrel (Sciurus aberti) on the south rim diverged after the canyon formed roughly 5 to 6 million years ago. The two forms differ in coat color and tail pattern, and the geographic barrier of the canyon has prevented gene flow long enough that some taxonomists treat them as separate species.
Reverse Evolution
/ ri-VURS ev-uh-LOO-shun / · Latin reversus meaning turned back and evolvere meaning unfold
Reverse evolution is the reappearance in a lineage of a trait resembling one present in an ancestor but lost in intervening generations, driven by new mutations or the reactivation of dormant genetic pathways rather than by literal reversal of prior mutations.
Dollo’s law, proposed by paleontologist Louis Dollo in 1893, states that evolution is irreversible because the probability of restoring a lost complex structure through the same sequence of mutations is vanishingly small. Apparent reversals do occur, but they typically arise through different genetic routes than the original trait used. Stick insects (order Phasmatodea) offer a well-studied case: a 2003 phylogenetic analysis by Michael Whiting and colleagues suggested that wings were lost and then regained multiple times across the group, though this interpretation remains debated.
In threespine sticklebacks (Gasterosteus aculeatus), freshwater populations repeatedly lose their bony armor plates through changes at the Ectodysplasin gene, and laboratory crosses can restore plate coverage, showing that the genetic machinery persists even after the trait disappears from a population.
Atavisms, individual organisms that express a trait absent in recent ancestors, offer visible evidence that ancestral developmental pathways can persist silently for millions of years. Humpback whales (Megaptera novaeangliae) occasionally develop small hindlimb projections, reflecting the retention of limb-development genes inherited from terrestrial ancestors.
Evolution can run backward like a film in reverse. Any reappearance of an ancestral-looking trait is built from the genetic and developmental material available in the current lineage, which differs from what the original ancestor possessed.
Cave populations of the Mexican tetra (Astyanax mexicanus) have independently lost eye pigmentation through mutations in different genes across separate cave systems. When researchers cross fish from different caves, some offspring regain functional eyes, demonstrating that the developmental pathways for eye formation remain present even after the visible structures are lost.